Wood grinding

ABSTRACT

The invention is concerned with the grinding of wood, and in particular to the production of wood pulp in the paper-making industry. The invention provides a method for the grinding of wood which includes the steps of: (a) urging wood against a grinding surface which is adapted for rotation by a source of power, (b) applying to the grinding surface a flow of cooling liquid, (c) maintaining the ratio of the pressure of the wood against the grinding surface to the rate of flow of coolant liquid at a substantially constant value, and (d) maintaining the energy supplied by the source of power per unit weight of ground wood produced at a substantially constant level. The invention also contemplates the provision of apparatus for the grinding of wood which comprises: (a) a grinding surface adapted to be rotated by a source of power, (b) means for urging wood against the grinding surface, (c) means for applying to the grinding surface a flow of cooling liquid, (d) means for maintaining the ratio of the pressure of the wood against the grinding surface to the rate of flow of coolant liquid at a substantially constant value, and (e) means for maintaining the energy supplied by the source of power per unit weight of ground wood produced at a substantially constant level.

. United States Patent I n j 3,693,891; Remm'er I [451 Sept. 26, 1972 [54] WO OD GRINDING I v I r "and in particular to the production of wood pulp in ml" m the paper-making industry. The invention provides a [72] Inventor i' g g g ga method for the grinding of wood which includes the a I steps of:' (a)- urging-wood against a grinding surface [22] I Filed: I June 24, 1971 Y l which is adapted for rotation by a source of power, x (b) applying to the grinding surface a flow of cooling 1 A 156328 1 liquid, (c) maintaining the ratio of the pressure of the r I wood against the grinding surface to the rate of flow [52.] us. .01. ..241/15, 241/28, 241/33, of coolant liquid at a substantially constant value, and

. t I I 241/34, 241/282 (d) maintaining the energy supplied by the'source of [51,] Int; Cl.'.. ..B02c 25/00 power per unit weight of ground wood produced at a I 153 n w g s m zu [15, 27, 23, 30, 33, 34, 'substantially constant level. The invention also con- 24 35, 3 41, 3, 5, 5, 230, 232 templates the provision of apparatus for the grinding of wood which comprises: (a) a grinding surface I 5 mu en I i adapted tobe rotated by a source of power, (1)) means I for urging wood against the grinding surface, (c) UNITED STATES PATENTS 1 means for applying to the grinding surface a flow of cooling liquid, (d) means for maintaining the ratio of fig; the pressure of the wood against the grinding surface 3606l77 9/19. I iag'gigi 41/282 x I to the rate of flow of coolant liquid at a substantially 316541075 4/1972 Keyes et a1.......::: 241/36 x i and (e) M maimaming [energy supplied by the source of power per unit Primary Examiner,Granville Y. Custer, Jr. 1 3 22 3 52? woo produced at a substamlauy [I57] I ABSTRACT 37 Claims, 2 Drawing Figures Th im n s c n rn wi h. shesrissiinspiw d 14%??- {Pop-r "I-Ii.

D I I 4 n I )CONTROL P/Q wooo I Q IN l SUPPLY COOLANT l ""1'Z ILlQUlD I l E I I 2- i 1/ GRlNDiNG i l c we -i "Wa -I I12 I ,7 CoNTR oLK I l i GROUND I I I I 8 W000 I l I PRODUCT I I I I I IL .1 l

v I J\ COMPUTER "71 I PATENTEU EP m2 3 693,891

SHEET 1 OF 2 INVENTOR. NORTON 5 EEMMEI? iiww A TTOENEY woon GRINDING This invention relates to the grinding of wood by urging or forcing the wood against a rotating grinding wheel. In particular, the present invention is concerned with control of the quality of ground wood produced in such a manner, and to control of various factors which influence the properties of the resultant product.

The grinding of wood to produce ground pulp is a well-known-process, which finds particular application in the paper-making industries. Differing types of grinding apparatus are utilized, employing various ways of feeding and urging the wood against the rotating grinding wheel. One of the most common and popular types has one or more pockets adjacent the grinding wheel to which the wood is supplied, and which force the wood against the grinding wheel using hydraulic pressure. This type of wood grinder does not have a continuous constant rate of grinding, since the pocket or pockets periodically must be recharged with wood which is to be ground. Another type of grinding apparatus utilizes a continuous chain to feed the wood and urge it against the rotating grinding wheel; an endless chain with projections thereon engaging the wood and bringing it to the grinding wheel. A third type of grinder is of the ring-type in which the rotating grinding wheel is eccentrically mounted within a rotating ring; the wood being forced between the wheel 'and the ring. In each case, fluctuations in the rate of grinding are produced by variations in the amount of wood contacting the grinding wheel, the pressure forcing it against the grinding wheel, and the speed at which the grinding wheel is rotating at any given instant.

The grinding wheels of such wood pulp grinders are frequently driven using electrical power, and the amount of energy required to grind the wood depends upon several factors. For example, the type and density of wood being ground, the rate at which it is being ground, the pressure with which the wood is being urged against the surface of the grinding wheel, and the rate of flow of cooling liquid wetting the surface of the grinding wheel all influence the. electrical power required to rotate the grinding wheel. These factors not only have a bearing on the energy required to rotate the grinding wheel, but also affect the quality of the product, namely the ground wood. Another factor which is known to affect the quality of the product is the temperature of the wood in the grinding zone during the grinding process, and this temperature is influenced by the rate of flow and temperature of the cooling liquid as well as by other factors.

A continuing problem has been the effective control of the quality of the ground wood produced by this type of apparatus, and various attempts have been made to control the several factors mentioned above in an effort to obtain a product of consistent quality. in addition, economic use of electrical or other energy used to rotate the grinding wheels is an important consideration. Wood grinding mills frequently utilize a number of grinders operating simultaneously, and it is advantageous to maintain a uniform consumption of power, particularly in the case of electrically driven grinders. Accordingly, individual grinders in the past have been controlled so as to maintain the total rate of energy consumption by the grinders at a substantially I lar installation and application.

constant value. For example, with electrical power, it is desirable to maintain the rate of power consumption at a predetermined constant number of kilowatts. It has been customary to control the power required for grinder operation, in each individual grinder, by varying the force with which the wood is urged against the grinding wheel. However, this produces a fluctuation in the rate at which the wood is being ground, and this arbitrary control of individual grinders also results in variation in the quality of the ground wood product.

The present invention flows from a realization that the quality of the wood pulp produced by a wheel grinder (having a grinding wheel rotated by a source of power, wetted by a flow of cooling liquid, and against which the wood is forced), can be closely controlled by maintaining the energy per unit weight of ground wood supplied by the power source at a substantially constant value. At the same time, the ratio of the pressure used to urge the wood against the grinding wheel compared to. the flow of cooling liquid is maintained at a substantially constant value. These constant values can be determined by calculation and/or experiment to best meet the power and quality requirements of a particu- The expression, energy consumed per unit weight of ground wood, will hereinafter be represented by the symbol Ew. It is apparent that Ew can be determined from and is obtained by dividing the power consumption (conveniently expressed as kilowatts) by the rate of grinding (conveniently expressed as air-dried tons per hour). Using the symbols E for power consumption and R for the rate of grinding, the expression becomes:

It now has been discovered that the energy consumed, Ew, is a function, among other things, of the pressure (denoted by the symbol P) with which the wood is urged against the grinding wheel, and also the rate of flow of cooling liquid (denoted by the symbol Q). The pressure of the wood applied against the grinding surface and the flow rate of cooling liquid themselves are adjusted to take into account the state of the grinding wheel surface and the temperature of the grinding wheel/wood interface. Adjustments to the value of P and Q take into account these two variables, and it is known that these variables play a significant part in determining the quality of the ground wood product.

It is desirable to achieve a rate of grinding which is consistent with and provides a given quality of product at a constant energy consumption level. According to this invention, it has been found that a desired rate of grinding which provides a product of given quality and produced at a specified power consumption level may be achieved by maintaining the ratio of pressure, P, to rate of flow of cooling liquid, Q, at a preselected constant value. Expressed as a mathematical equation:

where K stands for a constant.

This invention, therefore, provides a method for the grinding of wood which includes the steps of:

a. urging wood against a grinding surface which is adapted for rotation by a source of power,

b. applying to the grinding surface a flow of cooling liquid,

c. maintaining the ratio of the pressure of the wood against the grinding surface to the rate of flow coolant liquid at a substantially constant value, and d. maintaining the energy supplied by the source of power per unitweight of ground wood produced at a substantially constant level.

In addition, this invention contemplates the provision of a wood grinding assembly which includes:

a. a grinding surface adapted to be rotated by a source of power,

b. means forurging wood against said grinding surface,

c. means for-applying to said grinding surface a flow of cooling liquid, V

d. meansfor maintaining the ratio of the pressure of the wood against the grinding surface to the rate of flow of coolant liquid at a substantially constant value, and

e. means for maintaining the energy supplied by the sourcev of power per unitweight of ground wood produced at a substantially constant level.

Following the teachings of this invention, Ew and thus the quality of the wood product can be maintained at a consistent level by maintaining the ratio of pressure to flow rate of cooling liquid, P/Q, at a substantially constant value. According to this invention,.rnaintaining ,the ratio of the. pressure P to the rate of flow of cooling liquid Q at a substantially constant value will.

result in the production of ground wood pulp of generally uniform quality. Expressed mathematically:

Ew w P/Q Therefore,Ew can be maintained substantially constant by maintaining the ratio of P/Q at a substantially constant value. Variation in one of the factors affecting these values also will affect the value of other factors, and it is desirable to maintain the ratio as near as possible to its preselected and desired value. In this way the quality of the ground wood product can be predetermined and substantially maintained, and it is possible to produce a wide range of product characteristics at preselected energy levels per unit-weight of wood by suitable variations in the factors having a bearing on the value of the ratio P/ Q a A desired rate of grinding (denoted by the symbol R) may be achieved with a given quality and at a-desired power consumption levelby maintaining the ratio of pressure P to the rate of flow of cooling liquid at a preselected constant value. It will be apparent that the he wood of grinding R can be affected and controlled by changes in the pressure P forcing the wood against the grinding surface. To increase the rate of grinding P is increased,and to decrease the rate of grinding P is reduced. At the same time, in accordance with this invention, the rate of flow of cooling liquid, Q, would be altered so as to keep the ratio P/Q substantially constant. Appropriate adjustment would be made to the power consumption E in order to maintain Ew at a constant value and thus the quality of the ground wood at a consistent level.

In order to control thewood grinding process in accordance with the teachings of this invention, the rate of grinding R, the pressure forcing the wood against the grinding wheel-P, and the rate of. flow of cooling liquid Q are constantly monitored, as is the rate of consumption of power, E. The rate of grinding, R, may be measured by several different methods. For example, where hydraulic pressure is utilized to urge the wood against the grinding wheel the rate of travel of the hydraulic piston can be measured and translated to a volume measurement utilizing a knowledge of the density of the wood being ground. An alternative way of determining the rate of grinding is by weighing the total effluent of water and pulp on a continuous basis and'subtracting the weight of coolingfluid being applied, with suitable correction for the time lag. Another method is the incorporation of weight sensing devices in the magazines of grinders which usehydraulic pressure, and from them obtaining a direct reading of tonsper hour being ground. The rate. of flow of the coolant liquid, Q, is relatively easily obtained throughthe use of a flow meter. The hydraulic-pressure, P, beinglapplied to the wood to force it against the grinding surface of the grinding wheel is obtained through the use of a pressure gauge.

The rate of grinding, R, is continuously monitored, and the ratio of P to Q is continuously recorded. This ratio is kept substantially constant by varying either P or Q. The choice will depend. upon the individual operating apparatus and conditions. Since the pressure being applied to the wood is known and can be controlled, and the quantity of coolant liquid being applied to the grinding ,wheel is also known and can be controlled, the ratio of P/ Q can'be calculated and kept substantially constant through alteration in the value of either P or Q. Since the energy supplied to the grinding wheel per unit weight of wood ground, Ew, is equal to the power consumption, E, divided by the rate of grinding, R, and since the rate of grinding can be determined by either computing it or measuring it directly, all of the factors required to control the variables of the process in accordance with this invention are known.

Since it is known that a relationship between the energy" consumption, the pressure applied to the wood, and the quantity of coolant liquid can be determined. This relationship is expressed by the equation Ew a mK where a and m are constants. It is known that Ew= E/R and Ew w P/Q.

Since K P/Q It follows that The units of the variables expressed in the above equation are normally given as follows:

P=pressure in p.s.i. X 10 Q= lbs. of shower water per minute Ew.= kilowatt-hours per air-dried ton of output.

The value of the constants a and m and the ratio P/ Q for any particular case may be determined by calculation and/or experiment, having regard to the design and process limitations of the equipment. Examples of fac' tors which influence the value of the constants and the ratio P/Q are: (1') the speed of the grinding wheel, (2)

the available motor capacity, (3) the available hydraulic pressure, and (4) the size of the wood pockets. The temperature at the wood grinding surface also imposes some limitations on the pressure and the rate of flow of coolant liquid, since above particular temperature levels the wood becomes discolored and burned. This discussion also has been based on the assumption that the cooling liquid was just that, a shower of cool liquid derived from a source of relatively consistent temperature. If substantial variations in the temperature of the cooling liquid occur, this is an additional factor which it may be necessary to consider, and which should be monitored and included in the factors enumerated in the control of the process. Such factors and limitations are well known and within the operating experience of mill operators handling wood grinding apparatus. In addition, there also are economic factors which should be given consideration, such as employment of excessive volumes of cooling liquid, since it is generally practical to operate with liquid quantities which maintain a desired end product consistency.

The invention now will be described in further detail with reference to a particular embodiment showing a typical wood grinder, and the application of the principles of the present invention to the operation thereof. It should be recognized that this particular example is included herein to facilitate a ready understanding of the invention and one way in which it may be put into practice, and the presence of the example is not to be construed as limiting the scopeof the invention and the apparatus to which it may be applied.

The specific embodiment is illustrated in and will be described with reference to the accompanying drawings, in which:

FIG. 1 is a view, in section, of a typical two-pocket wood grinder.

FIG. 2 is a diagrammatic representation of a typical system for controlling the grinder shown in FIG. 1, utilizing the principles of this invention.

Referring to the drawings, F IG. 1 shows a two-pocket grinder having a rotatable grinding wheel 1 with wood pockets 2 on opposite sides of the grinding wheel. Wood in the form of logs 3 in eachpocket 2 is urged against the grinding wheel 1 by a piston 4 actuated by a hydraulic pressure cylinder 6. The hydraulic cylinders 6 are conventionally operated, often by water pressure at two different levels; a low-pressure high-volume supply of the order of about 150 lbs. per square inch to both retract and advance the piston rapidly for initial compression of a new charge of logs, and a high-pressure low-volume supply which may have a pressure of up to 500 to 600 lbs, per square inch for use during the actual grinding process. The flow of hydraulic liquid to and from the cylinder 6 is controlled by a conventional valve arrangement (not shown).

Cooling liquid, normally water, is supplied to the grinding wheel 1 from multi-orifice shower pipes 7 located at predetermined positions on the top and at the bottom of grinding wheel 1. The coolant performs several functions; the cleaning of the grinding wheel and extraction of heat generated by the grinding process, as well as serving as a vehicle for the discharge of product to a collection flume 8. The level of wood pulp and water in the flume 8 may be controlled by a dam 9 of variable height.

In the schematic diagram of FIG. 2, the same reference numerals are utilized to identify like components of the apparatus in FIG. 1, even though the elements are illustrated in block diagram form. Grinding wheel 1 is driven by an electrically powered motor 12. A signal of the power consumption of the motor, K in, in kilowatts is fed into a process control computer 11. Provision also is made for a control line from the process controller to the motor, K out, as indicated.

The pressure being applied by each hydraulic piston 4 is measured and passed to the process control computer 11, P in, and a valve 14 responsive to an output signal, P out, from the computer control can vary the pressure being applied by hydraulic piston 4.

The quantity of cooling water flowing through shower pipes 7 is measured by a flow meter, and is converted to a flow rate which is passed to the process control computer 1 1, Q in. The flow rate can be controlled through operation of a control valve 13, in response to signals from the computer 11, 0 out.

The rate of grinding, R, may be determined by. a number of different methods. A transducer 10 which can determine the weight of the charge at any given time and thus'its rate of consumption from the pocket 2 may be'incorporated as a control sensing element, and the rate of grinding computed directly. Alternatively, the dam 9 may discharge to a weighing system 15, and knowing the rate of flow of cooling liquid, the rate of grinding may be computed. The rate of grinding also may be determined from the. rate of travel of each hydraulic piston 4. For this purpose, referring back to FIG. 1, a rack 21 is connected to and travels with each piston 4, and a microswitch 22 with a cam-follower 23 engaging the rack 21 is periodically actuated by movement of the rack. The number of times each microswitch 22 is actuated is, therefore, a measure of the quantity of wood being ground, and an appropriate quantity signal can be sent to the computer 11 (which includes a clock timer) to calculate the rate of grinding, R, convenient units for which are tons per hour. Since the microswitch 22 is actually responsive to the volume of wood rather than to its weight, it also is necessary to know or assume a wood density value in'order to calculate the rate of grinding. A pressure switch (not shown) would be included in the microswitch circuit so that the travel of the rack 21 is only measured when the hydraulic pressure in the cylinders 6 is above a predetermined value, for example pounds per square inch, so that movement of the piston 4 is only recorded during grinding and not while the pocket is being recharged. The process control computer 11 is programmed to calculate an average rate of grinding for the two pockets 2.

The rate of grinding, R, is periodically obtained and the ratio of P/Q is continuously monitored. This ratio is kept substantially constant by varying either P or Q, depending upon the particular control program being utilized in connection with the apparatus. The hydraulic pressure P and the coolant flow rate Q is determined from signals fed to the computer and the ratio of P to Q is periodically determined. The value of this ratio is compared with a predetermined desired value, and the computer can feed back a signal to the pressure control valve 14 or to the flow control valve 13 so as to alter either P or Q and maintain the ratio at the predetermined desired value.

black spruce, and the pockets of the grinding apparatus in which the wood was placed were 50 inches long and 36 inches high. v V

The relationship between the energy consumed-per unit of ground'product, the pressure applied to the wood being ground, and the rate .of coolant flow is given by the expression.

where a and'm are constants; P= pressure in p.s.i. X Q= lbs. of shower water per minute Ew kilowatt-hours per air-dried ton of output.

From experiment it was determined 'that the constants for the general relationship Ew a mK had values as follows: I

v a =l 5,461

and 6.335 K 6.530.

If P/Q K 6.4, then for any rate of grinding desired within the control range, Ew has a value of about 1,160 kilowatt-hours per ton. v

For any type of grinder, the general relationship may be utilized. It is only necessary to be able to monitor the rate of grinding R, either Por Q, and the power consumption E of the motor driving the grinding wheel. If the power E is known, Ew can be computed knowing R and Q. The desiredvalue of Ew can be achieved by adjusting Q, and making appropriate'changes'to P so as to keep the ratio P/Q at the desired constant value.

The inter-relationship between the various factors and the control of these factors to produce thedesired result'is most easily done by monitoring and adjusting the several factors which make up the total process and by maintaining a specific relationship between them with the aid of a computer, in the broadest sense of the word. Suitably, this may be an electronic digital -or anolog computer of the process control type, which has capabilities of determining the status at any given interval of certain aspects of the process, performing computations relating to the interdependence of the several aspects, and which is capableof transmitting a signal to apparatus which signal can alter the status of these factors. in a particular case, it may be easier to vary the. rate of flow of cooling fluid Q, than it is to vary the force P which is urging the wood against the grinding wheel, and therefore'the ratio of P/Q may be more easily maintained as a constant value through control of the flow of the cooling liquid.

In the case of a continuous pulp Wood grinder in which the wood is urged against the grinding wheel by means of chains having projections engaging the wood,

it is" not easy to measure the grinding force P. In this particular case, it may be preferred to control the energy being supplied to the grindingwheel motor so as to maintain Ew substantially constant. A signal indicating 5 thekilowatt power consumption of the motor, together with signals of the rate of grinding and the quantity of coolant liquidcan be fed to a computing device, and the motors speed and the rate of cooling can be varied to maintain the required relationship between these factors.

Although the word grinding has been used throughout this specification, and the invention has been described with reference to apparatus using a grinding wheel, it will be apparent that it also is applicable to wood processing equipment utilizing a rotating wheel which does not have an actual grinding surface in the strictest meaning of the word but rather which may have a chipping, chewing or cutting action which 20, reduces the wood to small pieces and chunks ready for further treatment as pulpwood. The word grinding wherever used throughout the specification and claims of this application is to be construed as having a meaning which extends to cover the action of reducing wood to small pieces, chips and chunks, whether or not the actualnworking surfaceis that of grinding wheel in the conventional and narrower sense of the word.

The teaching of the grinding method of the present invention contemplates the effective grinding of materials other than wood. Other fibrous materials, for

example, having a consistency, composition and hard ness factor such that they would respond to grinding treatment by the apparatus of the present invention might usefully and economically be subjected to the methodherein. The same controls would be used to produce the most efficient grinding and the selection of operable materials will be apparent to the man skilled in the art.

lclaimf 1. A method of grinding wood, which includes th steps of:

a. urging wood against a grinding surface which is adapted for rotation by a source of power; b. applying to .the grinding surface a flow of cooling liquid; i

c. maintaining the ratio of the pressure of the wood against the grinding surface to the rate of flow of coolant liquid at a substantially constant value; and

d. simultaneously maintaining substantially constant the energy supplied by he source of power per unit weight of ground wood produced.

2. In a method of grinding wood which includes urging wood against a grinding-surface, which surface is adapted for rotation by a source of power and has a flow of cooling liquid applied thereto, the steps of:

. a. determining the rate factor at which wood is being ground;

b. determining the factor of the rate at which power is being consumed by said source of power; c. determining the pressure factor urging the wood against said grinding surface; d. determining the rate of flow factor of said coolant liquid; e. varying at least two of the determined factors of rate of power consumption, rate of flow of coolant liquid, and pressure, to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow of coolant liquid at a predetermined and substantially constant value, and to maintain at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.

3. A method of grinding wood as defined in claim 2 which includes the steps of:

e. varying said pressure factor so as to maintain the ratio between the pressure and the rate of flow of coolant liquid at a substantially constant selected value; and

f. simultaneously maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.

4. A method of grinding wood as defined in claim 2 which includes the steps of:

e. varying the rate of flow of coolant liquid factor so as to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow at a substantially constant selected value; and

f. simultaneously maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.

5. A method of grinding wood as defined in claim 2 which includes the steps of:

e. varying the rate of flow of coolant liquid factor so as to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow at a substantially constant selected value; and i v 'f. varying the rate of power consumption factor to maintain at a substantially constant selected level the energy supplied by the source of power per unit weight of ground wood produced. a

6. A method of grinding wood as defined in claim 2 l which includes the steps of:

e. varying said pressure factor so as to maintain the ratio between the pressure and the rate of flow of coolant liquid at a substantially constant selected I where a and m are constants.

8. A method of grinding wood as defined in Claim 2 wherein the several factors are determined and controlled with the aid of a computer.

9. A method of grinding wood as defined in claim 3 wherein the several factors are determined and controlled with the aid of a computer.

10. A method of grinding wood as defined in claim 4 wherein the several factors are determined and controlled with the aid of a computer.

i 11. A method of grinding wood as defined in claim 5 wherein the several factors are determined and controlled with the aid of a computer.

g 12. A method of grinding wood as defined in claim 6 wherein the several factors are determined and controlled with the aid of a computer.

13. A method of grinding wood as defined in claim 7 wherein the several factors are determined and controlled with the aid of a computer.

14. A method of'grinding wood as defined in claim 8 wherein said computer is an electronic digital computer adapted for process control.

15. A method of grinding wood as defined in claim 9 wherein said computer is an electronic digital computer adapted for process control.

16. A method of grinding wood as defined in claim 10 wherein said computer is an electronic digital computer adapted for process control.

17. A method of grinding wood as defined in claim 11 wherein said computer is an electronic digital com puter adapted for process control.

18. A method of grinding wood as defined in claim 12 wherein said computer is an electronic digital computer adapted for process control.

19. A method of grinding wood as defined in claim 13 wherein said computer is an electronic digital computer adapted for process control.

20. A wood grinding assembly including:

a. a grinding surface adapted to be rotated by a source of power,

b. means for urging wood'against said grinding surface,

c. means for applying to said grinding surface a flow of cooling liquid,

d. means for maintaining the ratio between the pressure of the wood against the grinding surface and the rate of flow of coolant liquid at a substantially constant value, and Y e. meansfor maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.

21. A wood grinding assembly including;

a. a grinding surface adapted to be rotated by a source of power,

b. means for urging wood against said grinding surface,

c. means for applying to said grinding surface a flow of cooling liquid,

d. means for determining the rate at which wood is being ground,

e. means for determining the rate of power consumption by said source of power,

f. means for detennining the pressure urging the wood against said grinding surface,

g. means for determining the rate of flow of said coolant liquid, and

h. means for varying at least two of (i) the rate of power consumption, (ii) the rate of flow of coolant liquid and (iii) said pressure to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow of coolant liquid at a substantially constant value, and to maintain at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.

22. A wood grinding assembly as defined in claim 21 including:

b. means for varying said pressure so as to maintain the ratio between the pressure and the rate of flow of coolant liquid at a substantially constant value, and i. means for maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.

23. A wood grinding assembly as defined in claim 21 7 including: 7 i

h. means for varying the. rate of flow of coolant liquid so asto maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow at a substantially constant value, and i i. means for maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced. 24. A wood. grinding assembly as defined in claim 21 including:

h. means for varying the rate of flow of coolant liquid so as to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow at a substantially constantv value; and r i. means for varying the rate of power consumption to maintain at a substantially constant levelthe energy supplied by the source of power per unit weight of ground wood produced. 25. A wood grinding assembly as defined in claim 21 including:

12 of ground wood produced. 26. A wood grinding assembly as defined in claim 20 wherein said means defined i (d) and (e) comprise a i and (i) comprise a computer.

h. means for varying said pressure so as to maintain the ratio between the pressure-and the rate of flow of coolant liquid at a substantially constant value; and

i. means for varying the rate of power consumption to maintain at a substantially constant in the energy supplied bythe source of power per unit weight 31. A wood grinding assembly as definedin claim 25 wherein said means defined in (d), (e), (f), (g), (h) and (i) comprise a computer.

32. A wood grinding assembly as defined in claim 26 wherein 'said computer is an electronic digital computer adapted for process control. a

33. A wood grinding assembly as defined in claim 27 wherein said computer is an electronic digital computer adapted for process control.

34. A wood grindinga'ssembly as defined in claim 28 wherein said computer is an electronic digital comt d t d f control. s sl is 3303a g rirl di n g sembly as defined in claim 29 wherein said computer is an electronic digital computer adapted for process control.

36. A wood grinding assembly as defined in claim 30 wherein said computer is an electronic digital computer adapted for process control.

37. A wood grinding assembly as defined in claim 31 wherein said computer is an electronic digital computer adapted for process control.

V I V v 

2. In a method of grinding wood which includes urging wood against a grinding surface, which surface is adapted for rotation by a source of power and has a flow of cooling liquid applied thereto, the steps of: a. determining the rate factor at which wood is being ground; b. determining the factor of the rate at which power is being consumed by said source of power; c. determining the pressure factor urging the wood against said grinding surface; d. determining the rate of flow factor of said coolant liquid; e. varying at least two of the determined factors of rate of power consumption, rate of flow of coolant liquid, and pressure, to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow of coolant liquid at a predetermined and substantially constant value, and to maintain at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 3. A method of grinding wood as defined in claim 2 which includes the steps of: e. varying said pressure factor so as to maintain the ratio between the pressure and the rate of flow of coolant liquid at a substantially constant selected value; and f. simultaneously maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 4. A method of grinding wood as defined in claim 2 which includes the steps of: e. varying the rate of flow of coolant liquid factor so as to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow at a substantially constant selected value; and f. simultaneously maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 5. A method of grinding wood as defined in claim 2 which includes the steps of: e. varying the rate of flow of coolant liquid factor so as to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow at a substantially constant selected value; and f. varying the rate of power consumption factor to maintain at a substantially constant selected level the energy supplied by the source of power per unit weight of ground wood produced.
 6. A method of grinding wood as defined in claim 2 which includes the steps of: e. varying said pressure factor so as to maintain the ratio between the pressure and the rate of flow of coolant liquid at a substantially constant selected value; and f. varying the rate of power consumption to maintain at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 7. A method as defined in claim 2 wherein the relationship between said factors is controlled according to the equation: Ew a + m(P/Q) where a and m are constants.
 8. A method of grinding wood as defined in claim 2 wherein the several factors are determined and controlled with the aid of a computer.
 9. A method of grinding wood as defined in claim 3 wherein the several factors are determined and controlled with the aid of a computer.
 10. A method of grinding wood as defined in claim 4 wherein the several factors are determined and controlled with the aid of a computer.
 11. A method of grinding wood as defined in claim 5 wherein the several factors are determined and controlled with the aid of a computer.
 12. A method of grinding wood as defined in claim 6 wherein the several factors are determined and controlled with the aid of a computer.
 13. A method of grinding wood as defined in claim 7 wherein the several factors are determined and controlled with the aid of a computer.
 14. A method of grinding wood as defined in claim 8 wherein said computer is an electronic digital computer adapted for process control.
 15. A method of grinding wood as defined in claim 9 wherein said computer is an electronic digital computer adapted for process control.
 16. A method of grinding wood as defined in claim 10 wherein said computer is an electronic digital computer adapted for process control.
 17. A method of grinding wood as defined in claim 11 wherein said computer is an electronic digital computeR adapted for process control.
 18. A method of grinding wood as defined in claim 12 wherein said computer is an electronic digital computer adapted for process control.
 19. A method of grinding wood as defined in claim 13 wherein said computer is an electronic digital computer adapted for process control.
 20. A wood grinding assembly including: a. a grinding surface adapted to be rotated by a source of power, b. means for urging wood against said grinding surface, c. means for applying to said grinding surface a flow of cooling liquid, d. means for maintaining the ratio between the pressure of the wood against the grinding surface and the rate of flow of coolant liquid at a substantially constant value, and e. means for maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 21. A wood grinding assembly including: a. a grinding surface adapted to be rotated by a source of power, b. means for urging wood against said grinding surface, c. means for applying to said grinding surface a flow of cooling liquid, d. means for determining the rate at which wood is being ground, e. means for determining the rate of power consumption by said source of power, f. means for determining the pressure urging the wood against said grinding surface, g. means for determining the rate of flow of said coolant liquid, and h. means for varying at least two of (i) the rate of power consumption, (ii) the rate of flow of coolant liquid and (iii) said pressure to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow of coolant liquid at a substantially constant value, and to maintain at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 22. A wood grinding assembly as defined in claim 21 including: h. means for varying said pressure so as to maintain the ratio between the pressure and the rate of flow of coolant liquid at a substantially constant value, and i. means for maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 23. A wood grinding assembly as defined in claim 21 including: h. means for varying the rate of flow of coolant liquid so as to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow at a substantially constant value, and i. means for maintaining at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 24. A wood grinding assembly as defined in claim 21 including: h. means for varying the rate of flow of coolant liquid so as to maintain the ratio between the pressure urging the wood against the grinding surface and the rate of flow at a substantially constant value; and i. means for varying the rate of power consumption to maintain at a substantially constant level the energy supplied by the source of power per unit weight of ground wood produced.
 25. A wood grinding assembly as defined in claim 21 including: h. means for varying said pressure so as to maintain the ratio between the pressure and the rate of flow of coolant liquid at a substantially constant value; and i. means for varying the rate of power consumption to maintain at a substantially constant in the energy supplied by the source of power per unit weight of ground wood produced.
 26. A wood grinding assembly as defined in claim 20 wherein said means defined i (d) and (e) comprise a computer.
 27. A wood grinding assembly as defined in claim 21 wherein said means defined in (d), (e), (f), (g) and (h) comprise a computer.
 28. A wood grinding assembly as defined in claim 22 wherein said means defined in (d), (e), (f), (g), (h), and (i) comprise a computer.
 29. A wood grinding assembly as defined in claIm 23 wherein said means defined in (d), (e), (f), (g), (h), and (i) comprise a computer.
 30. A wood grinding assembly as defined in claim 24 wherein said means defined in (d), (e), (f), (g), (h), and (i) comprise a computer.
 31. A wood grinding assembly as defined in claim 25 wherein said means defined in (d), (e), (f), (g), (h) and (i) comprise a computer.
 32. A wood grinding assembly as defined in claim 26 wherein said computer is an electronic digital computer adapted for process control.
 33. A wood grinding assembly as defined in claim 27 wherein said computer is an electronic digital computer adapted for process control.
 34. A wood grinding assembly as defined in claim 28 wherein said computer is an electronic digital computer adapted for process control.
 35. A wood grinding assembly as defined in claim 29 wherein said computer is an electronic digital computer adapted for process control.
 36. A wood grinding assembly as defined in claim 30 wherein said computer is an electronic digital computer adapted for process control.
 37. A wood grinding assembly as defined in claim 31 wherein said computer is an electronic digital computer adapted for process control. 